Acoustic apparatus



Oct. 26, 1937. H. F. OLSON ACOUSTIC APPARATUS Fild Dec. 20, 1934 \\\\\\\\\\\\\\\\\\\\\\\\\\kl\\\\\\\\\\\\\\\\\\\\\\\\\\\/ a a 7. 1/ 4/ MW M rw INVENTOR ATTORNEY Patented Oct. 26, 1937 UNIV-TED STATES.

ACOUSTIC APPARATUS Harry I. Olson, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application December .20, 1934, Serial No. 758,374 2 Claims. (oi. 181- 31) This invention relates to acoustic apparatus. More specifically it relates to apparatus for the elimination of cabinet resonance resulting from a loudspeaker or the like being enclosed in a 5; cabinet such as a radio cabinet.

The usual type of .box' enclosing the back of loudspeakers'coristitutes an acoustic impedance of such nature that the sign of the acoustic reactance at the loudspeaker changes from plus to minus or vice versa, as the sound being reproduced passes through certain frequencies. At these frequencies which are referred to as the natural resonant periods of the system, the acoustic output of the loudspeaker and associated system is considerably greater than at frequencies on either side of the resonant frequencies. This effect leads to non-uniform response over the frequency range and to a decrease in the fidelity of the system.

It is an object of the invention to improve the response over the frequency range of a loudspeaker by changing the type of acoustic impedance into which the back of the loudspeaker feeds. According to the invention the acoustic impedance into which the back of the loudspeaker feeds is made such that sound will be transmitted therethrough without attenuation. Other objects of the invention will become apparent upon reading the following specification and appended claims.

An approved form of the invention is illustrated in the accompanying drawing in which Fig. 1 illustrates diagrammatically an arrangement for the elimination of resonance frequencies in the box enclosing the back of a loudspeaker diaphragm; Fig. 2 is the electrical analog of the construction shown in Fig. 1, and Fig. 3 illustrates how the invention may be applied to a loudspeaker mounted in a radio cabinet.

In the modification illustrated by Fig. 1 the loudspeaker is mounted in a box which encloses the back of the diaphragm. For the purpose of convenience the driving mechanism has been omitted, but the diaphragm is illustrated at I0.

The box II which encloses the loudspeaker, contains a plurality of panels I2a, I22), I20, IZd, and He, arranged substantially parallel with the front wall 13 in which the diaphragm I0 is mounted. Each of the panels I20, I21), I20, IM and I26 contains an orifice Ma, Mb, I40, Md and Me. The panels are spaced from the wall 13 and from each other to form cavities C1, C2, C3, C4, C5. The orifices constitute acoustic inertance m1, m2, m3, m4 and ms. The cavities formed by the par- 65 tltions constitute acoustic impedances.

The operation of the apparatus illustrated in Fig. 1 can be determined by referring to the circuit of Fig. 2 which is the electrical analog of the apparatus of Fig. 1. In the electrical analog Z0 represents the'impedance of the opening in the back of the box into the surrounding air 5 and Z0 represents the total impedance of the loudspeaker. The shunt impedances C1, C2, C3, C4, C5 represent the acoustic capacitances of the cavities between partitions and the series imped- 10 ances m1, m2, ma, m4, m5 represent the acoustic inertance of the orifices. The structure as illustrated in Fig. 2, is an acoustic low pass filter.

The acoustic impedance of the orifices is given by equations 15 10.: Z1 -5 where l=density of air w=21rf c=conductivity of orifices 20 ,f=frequency and where R=radius of orifices l=length of orifices The acoustic impedance of the cavities is given by Z i s0 where A=velocity of sound V=volume of cavity the limiting values of no attenuation is determined It will be seen that it is possible to so choose the size of the orifices and cavities that sound will be transmitted through the back with no attenuation.

In this system there is no abrupt change in the acoustic impedance into which the back of the loudspeaker feeds and therefore the structure does not introduce irregularities in the response.

It may be desirable to locate the high fre- 5O quency cut off for the system within the region of response of the diaphragm. This should not cause any difficulty because at the higher frequencies the loudspeaker is very loosely coupled to the filter system. As the frequency increases any device enclosing a diaphragm has only a' slight effect upon the performance. That ls, the system enclosing the loudspeaker has the largest effect on the response at the lower frequencies.

' At low frequencies there is no abrupt change in the filter impedance and therefore there should be no discontinuities in the response of the loudspeaker due to the filterr system terminating the back of the speaker. V

V In Fig. 3 the system is shown applied to a radio 7 cabinet. The structure and arrangement of the panels and orifices is the same as in Fig. 1-. It is possible to put part of the radio receiving apparatus in the cavities but if this is to bedone, the cavities should be designed so-that they will have the proper volume when the apparatus is included therein. If the cabinet be placed against 'a wall or if the orifices in the outer'panel l2e be closed making the back of the box or cabinet completely closed;1Z-is infinite and therewill be a gradual attenuation of low frequencies but there will be no peaks or regions of undueresponse.

While I have shown and described an approved form of my invention it is to be understood that various modifications and changes may be made thereto withoutdeparting from the spirit of the invention and i therefore desire to be limited only by the scope of the appended claims.

I claim: Q

1; Acoustic apparatus comprising a loudspeaker including a diaphragm, a cabinet in which said loudspeaker is mounted, partitions in said cabinet; one of said partitions being arranged to form a chamber incommunication with the back of said diaphragm, the other partitions being arranged to form cavities with adjacent partitions, and a single orifice in each of said partitions' interconnecting said cavities, said orifices obstructed; the dimensions of said orificesand cavities being such that sound will be transmitted therethroughwithout attenuation. .i

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